The invention relates to a printing apparatus and a method of operation of such a printing apparatus, in particular, a transfer printing apparatus.
In such a printing apparatus (a part of which is shown in FIGS. 1 to C), inked ribbon 24 is positioned adjacent a substrate 22 to be printed, and ink is transferred from the ribbon 24 to the substrate 22 during a printing operation. It is well known to provide a pair of spools, one of which is a supply spool, from which unused inked ribbon is unwound, and the other of which is a take-up spool, on to which ribbon, from which ink has been removed, is wound. It is known to control the rotation of one or both of the spools, so as to appropriately control the transfer of the ribbon from the supply spool to the take-up spool, and in a reverse direction, if required. The ribbon thus advances in a ribbon path between the two spools.
A printhead 12 is provided adjacent the ribbon path, and is operable to transfer ink from the ribbon 24 to the substrate 22 to be printed. It is known to operate transfer printers in two different configurations, the first being “intermittent printing”, wherein, in use, the substrate 22 is held stationary during each printing operation, and the printhead 12 is moved relative to the substrate 22 to effect printing; and the second being “continuous printing”, in which the printhead 12 may be still or moved, while the substrate 22 is moved past the printhead 12, in a path adjacent the ribbon path, during printing operations. It is also known to provide a printing apparatus which is capable of being operable in either configuration.
Particularly in continuous printing configurations, it is known to provide a print roller 20 over which the substrate passes, and which may be driven to advance the substrate 22 past the printhead 12. The print roller 20 is positioned near to the printhead 12, on the opposite side of the ribbon 24 from the printhead 12, such that, during a printing operation, a portion of the substrate 22 and a portion of the ribbon 24 are positioned between the print roller 20 and the printhead 12.
During a printing operation, the printhead 12 moves in a direction which is substantially transverse to the direction of movement of the ribbon 24 and the substrate 22 in their respective paths, i.e., towards the ribbon 24 and the substrate 22. In a thermal printing apparatus, the printhead 12 includes a plurality of thermally energisable printing elements, each of which is selectively energisable, so as to enable a pixel of ink to be transferred to the substrate 22. The energisation of the printing elements is controlled so as to transfer ink to the substrate 22 in a desired pattern, for example to print data and/or an image on to the substrate 22. In order to complete the transfer process, the printhead 12 contacts an ink-free side of the ribbon 24, and presses the opposite, inked, side of the ribbon 24 against the substrate 22 so as to transfer pixels of ink, which have been heated, from the ribbon 24 to the substrate 22. The ribbon 24 is then separated from the substrate 22 by means of passing the ribbon 24 over a peel-off device, for example a roller 26.
The length of time that a pixel of ink is exposed to a heated printing element prior to the pixel being transferred from the ribbon 24 to the substrate 22 affects print quality, and there is an optimum heating period, to achieve a satisfactory transfer. If the ink is not heated for long enough before the printhead 12 contacts the ribbon to transfer the ink, the ink tends to adhere to the ribbon 24, rather than being transferred cleanly on to the substrate 22. Therefore, the resulting print can be patchy and inconsistent. If the ink is heated for too long before the printhead 12 contacts the ribbon 24, the ink can become too fluid, resulting in a blurred or smeared print.
Therefore, in order to obtain a satisfactory print quality, it is known in the art to adjust the position of the printhead 12 relative to the roller 20, for example horizontally or vertically, using shims. In printers where the printhead 12 is stationary relative to the print roller 20, it is known to mount the printer on a plate which can be adjusted substantially horizontally along a tangent to the print roller 20 struck at the top dead centre position E. FIGS. 1 to C show the printhead 12 in three different positions relative to the print roller 20. Of course, such manual adjustment relies on trial and error and thus is awkward and time consuming, and also wastes ink and substrate, as it is necessary to perform several test printing operations, to ensure that optimum print quality, or as close to optimum print quality as possible, is achieved. Once adjusted, the position of the printhead 12 relative to the print roller 20 is substantially constant, unless and until the position is manually readjusted to improve print quality.
Such adjustment of the position of the printhead 12 relative to the print roller 20 is a linear adjustment, in a direction which is substantially parallel with the ribbon path. Therefore, during a print operation, when the printhead 12 is moved “vertically” towards the print roller 20, i.e., in a direction which is transverse to the direction of movement of the ribbon 24 and the substrate 22, the printhead 12 may not be positioned at the top dead centre position E of the print roller 20.
The position of the printhead 12 relative to the print roller 20 is optimised for a particular average printing speed, i.e., an average ribbon speed. The position of the printhead 12 relative to the print roller 20 is generally constant, once set, and does not provide an optimum print quality at all ribbon speeds.
A top dead centre position E is a contact point on the circumference of the print roller 20, defined such that a distance moved by the printhead 12 between a first position and the contact point in the direction transverse to the direction of travel of the ribbon relative to the print roller, is a minimum. A tangent T to the circumference of the print roller 20 at the top dead centre position E subtends an optimal peel off angle φ relative to the ribbon 24 extending between the contact point and the peel-off roller 26. Aligning the printhead 12 with the top dead centre position E of the print roller 20 may be done by sight and manual adjustment by an operator. Of course, positioning of the printhead 12 is then operator dependent, and is likely to be at least slightly inaccurate.
Another known method of positioning the printhead 12 relative to the top dead centre position E of the print roller 20 is to provide a bracket, which can be mounted in or on a housing of the printing apparatus. However, errors may be incorporated into the position of the printhead 12 relative to the top dead centre position E during positioning of the bracket relative to the housing and the print roller 20.
A further known method of improving print quality at very high printing speeds (i.e., ribbon speeds of above approximately 500 mm/s) is to reduce the thickness of a protective coating on the heating elements of the printhead 12. This enables a steeper thermal gradient to be provided, i.e., warming and melting the ink more quickly than if a thicker coating were provided. A disadvantage of this method is that the useful life of the printhead 12 is decreased, owing to increased wear on the printhead 12 during printing operations. The less protected printhead is vulnerable to abrasion during repeated contact with the ribbon and indirect contact with the substrate and the print roller 20.